There are various types of processes as processes for producing reduced iron and ferroalloy and, among those processes, rotary-hearth-type processes are practically used as the processes having high productivity. A rotary-hearth-type process is the one mainly composed of an incinerator of the type wherein a refractory hearth having the toroidal shape of a disc (the center of which is lacked) rotates at a constant speed (hereunder referred to as “rotary furnace”) on rails under fixed refractory ceiling and sidewalls and is used for reducing metal oxide. The diameter of the hearth of a rotary furnace is 10 to 50 m and the width thereof is 2 to 6 m.
Powder containing metal oxide as raw material is mixed with a carbon-type reducing agent, is thereafter formed into raw material pellets, and is fed to a rotary furnace. An advantage of this is that the raw material pellets are laid close and still on the hearth and therefore the raw material pellets hardly collapse in the furnace. Another advantage thereof is that the problem of powdered raw material sticking to refractories is avoided and thus the bulk yield of the product is high. Further, this method has so far been employed increasingly since it has a high productivity and allows a less expensive carbon-type reducing agent and powdered raw material to be used.
In addition, a rotary hearth process is also effective for reducing steelmaking dust generated from a blast furnace, a converter and an electric arc furnace and thickener sludge generated from a rolling mill and removing impurities, thus is used also as a dust processing means, and is a process effective for the recycling of resources.
The operation of a rotary hearth process is outlined hereunder.
Firstly, a carbon-type reducing agent is mixed with metallic oxide comprising raw material of ore, dust and sludge in the amount required for reducing the metallic oxide and thereafter the mixture is formed into pellets several to over ten mm in size with a granulator such as a pan-type pelletizer while water is sprayed so that water content is about 10%. When the grain sizes of raw material ore and a reducing agent are large, they are crushed with a crusher such as a ball mill and thereafter kneaded and granulated.
The pellets are fed to a rotary hearth and laid in layers. The pellets laid on the hearth in layers are heated rapidly and incinerated for 5 to 20 min. at a high temperature of about 1,300° C. During the process, metal oxide is reduced by the reducing agent. mixed in the pellets and metal is formed. The metallization ratio after the reduction varies in accordance with metal to be reduced, and is 95% or more in the case of iron, nickel, or manganese and 50% or more even in the case of chromium that is hard to reduce. Further, when dust coming from the steelmaking industry is processed, impurities such as zinc, lead, alkali metal, chlorine, etc. are volatilized and removed in accordance with the reducing reaction and therefore the dust may easily be recycled to a blast furnace or an electric arc furnace.
In such a method of reducing metal and steelmaking dust with a rotary hearth, to form raw material and a reducing agent into pellets is an essential requirement. Therefore, it is important to put the mixture of metal oxide powder as raw material and a reducing agent in the state liable to be granulated in a preliminary treatment of the raw material and, for that purpose, various methods such as preliminary crushing of raw material, kneading in a ball mill and the like are employed.
As explained above, the reduction of metal oxide by the conventional rotary hearth method is excellent in productivity and production costs and is a method for producing metal economically. However, the problems of the method have been that: it is important to pelletize raw material and a reducing agent; thus it is necessary either to select raw material having an excellent granulation property or to improve a granulation property by installing an expensive crusher and crushing raw material. This incurs a large cost.
Actually, when ore such as iron ore is used as raw material, as the size of the raw material ore is large, the raw material has generally been crushed into several tens of microns in average diameter and thereafter granulated and produced into pellets. For that reason, the drawbacks of the method have been that: the cost of equipment for a crushing process is high; electricity is required for the operation of a crusher; and the maintenance accompanying the wear of the crusher is expensive.
There are some cases where pulverized raw material is used for saving a crushing cost. However, in such a case, the selection of raw material is limited and thus it has not been a commonly adaptable method. For solving the problem, it is effective to use powdered ore after wet separation, thickener dust from a blast furnace or a converter, sludge in a scale pit of a rolling process, precipitated sludge from a pickling process, or the like. However, in those cases too, a problem has been that a water content is sometimes excessive and therefore raw material is hardly granulated. In particular, such raw material is composed of very fine powder one to several tens microns in size and, as a result, it is likely to be slurry in the state of containing water, or, even after the raw material is dehydrated with a vacuum dehydrator or a filter press, water remains by 30 to 50% and, as it is, it is difficult to granulate because too much water is contained.
One of the methods for solving the problems is to granulate powder raw material after it has been dried completely with a heat source such as a hot blast. However, in this method, the powder raw material forms a pseudo-agglomeration during the drying and thus it is impossible to granulate it as it is. Therefore, the pseudo-agglomeration of the powder raw material has been crushed, formed in the state of fine powder again, thereafter mixed with other material and water, then granulated and, thereafter, reduced on a rotary hearth.
As a result, when the above method is employed, good compacts can be produced and, if the compacts are dried efficiently, metal oxide is reduced stably. However, by a conventional technology, a method for drying such compacts, in consideration of the physical state of the compacts, is not established sufficiently and it has merely been considered that only the drying of the compacts is enough. As a result, there have been problems in that the compacts crack and powdering occurs abundantly from the surfaces thereof. Furthermore, when drying conditions are worse, the compacts have sometimes exploded. Therefore, a means for solving the problems has been desired for a long time. Here, though a method of drying compacts beforehand is an effective means, the problem of the method is still that the method requires a heat source and a device for exclusively vaporizing the water content, even after the compacts are dried, by consuming a large amount of heat and is somewhat disadvantageous economically.
In particular, when dust or sludge generated in the metal refining industry and processing industry such as the steelmaking industry is collected from a wet dust collector or a settling tank, the dust or sludge contains a large amount of water, 90% at the most, and when it is attempted to reduce it by a rotary hearth process, the drying process and the succeeding crushing process have been problems.
As a method of using raw material without granulation in a rotary hearth process for solving the problems, for example, Japanese Unexamined Patent Publication No. H11-12619 discloses the method wherein raw material is formed into a tile shape with a compression molding machine and used in a rotary hearth process. Even in this method however, there have been problems in using raw material containing a large amount of water. That is, the problems have been that, as shown in Japanese Unexamined Patent Publication No. H11-12624: the water content in raw material is required to be adjusted to 6 to 18% and for that purpose a drying process is required in addition to a preliminary dehydration process; for that reason the complicated control of a water content is required. Further, another problem has been that, in order to charge such raw material, a complicated charging machine is required as shown in Japanese Unexamined Patent Publication No. H11-12621 and the maintenance cost of the equipment is high.
Further, when such a type of raw material, containing water, is directly charged into a high temperature rotary furnace, the problems have been that: explosion occurs due to a high water content in accordance with the evaporation of water; the raw material is pulverized and taken away with an exhaust gas; and thus the product yield deteriorates extremely. In a rotary hearth process, the temperature in the furnace is generally the lowest in the vicinity of a raw material inlet and about 1,150° C. to 1,200° C. even there. At such a high temperature, compacts in a wet state entail the problem of explosion accompanying sudden water evaporation. Even when an explosion does not arise, exfoliation occurs at the corner portions and the surface due to the eruption of water vapor. Therefore, even though reduction operation can be carried out, there have been the problems in that the bulk ratio of the reduced product decreases and the ratio of powder generated from the compacts increases. As a result, there remain the problems in that the ratio of powder metal that is lost in an exhaust gas increases relatively and the yield deteriorates.
The object of the present invention is to provide: a method for drying efficiently compacts comprising powder raw material containing water without the generation of explosion or cracking; a method for reducing compacts that makes it possible to reduce the compacts at a high yield without the generation of an explosion or the like even when compacts, in the state of powder containing water, are supplied directly to a rotary furnace and reduced; and a rotary-hearth-type metal reducing furnace therefor, those having not so far been realized by conventional methods.